1. Field of the Invention
The present invention relates generally to encoders and, more particularly, to an absolute position detection encoder usually known as an absolute encoder for reading a relative position between coder and detector by the use of an absolute pattern graduation scale of the coder, the relative position being output in the form of an absolute position.
2. Description of the Prior Art
Linear and rotary type encoders are used extensively as detectors for control over feed rates and stop positions of various actuators. Classified in another way, these encoders fall into two broad types: incremental and absolute. The incremental type encoder detects and outputs increases and decreases in relative movement between coder and detector. The absolute type encoder reads the relative position between coder and detector using a fully periodic array pattern (absolute pattern) scale of the coder. The relative position, after suitable processing, is output as an absolute position.
Conventional absolute encoders include one disclosed in Japanese Patent Pub. Laid-open No. 57-175211 and another in Japanese Utility Model Laid-open No. 60-152916. Encoders of this type work as follows. In an encoder setup, an absolute pattern on a coder is formed as a single track. Along this track are disposed a plurality of detectors at fixed intervals. A binary combination code of readings made magnetically or optically by the detectors relative to the pattern is translated into an absolute position.
Magnetically or optically run, these conventional absolute encoders require placing as many noncontact detectors as the number of bits in the above-mentioned combination code. The output signals from the detectors must be converted into binary numbers after waveform shaping into rectangular waves by an electrical circuit. However, when the output signals from the detectors are converted by waveform shaping into rectangular signals, the signal rise and fall time represents a finite time interval, such that temporal offset in timing is inevitably produced at each detector.
Therefore, when using a finer absolute pattern for raising the resolving power of the coder, timing synchronization with the rise time and the fall time of the output pulses from the detectors can be realized with difficulty, so that, in the course of the forward and reverse relative movements between the detector and the coder, the readout results of the detector outputs at the rise and fall positions may fail to give correct position codes, thereby producing errors in encoder outputs.
On the other hand, the minimum resolving power in the above-mentioned absolute encoder depends on the length in the track direction of the minimum reading unit constituting the absolute pattern. When it is desired to raise the resolving power to the utmost, the quantity of incident light into the detector may be diminished in an optical system encoder because of the reduced slit width of the coder, while the detected magnetic field may be weakened due to the reduced magnetization pitch of the coder in a magnetic type encoder. Thus it was felt that, in either case, limitations are imposed on improving the resolving power within the permissive size range of the coder.
In addition, with this type of absolute encoder, the minimum reading unit of the absolute pattern or the incremental pattern on the coder is formed by magnetized and non-magnetized portions in the case of the magnetic encoder and by transparent and opaque portions in the case of the optical encoder for correspondence with the binary signals. In generals, a signal identifying section, which identifies the signal contents of the minimum reading unit in the pattern to "0" or "1", is of the same length as the overall length in the track direction of the minimum reading unit, so that the boundary area between the portions produces noise to lower the readout accuracy. Therefore, with a magnetic encoder, the magnetized area, which proves to be the signal identification section, must be formed with high dimensional accuracy. With an optical encoder, when the minimum reading unit corresponding to one of the binary signals is formed by a through-hole in the coder, the mechanical strength of the coder is lowered when plural such units are formed so as to be contiguous to one another, so that it becomes necessary to reinforce the coder. Moreover, in both the magnetic and optical encoders, continuous units of a length up to N times the length of the minimum reading unit must be formed in the absolute pattern have the N-bit absolute position signal code contents, so that the larger the number N of the bits of the absolute signal code, the higher is the dimensional accuracy with which the pattern must be formed.